Precision ultrasound measurement for intraocular lens placement

ABSTRACT

Visual displays of the geometry and/or topography of a portion of the eye is obtained from data generated during a number of angularly spaced scans taken across a meridional coronal section or of a marginal sector of the anterior surface of the eye, the data being processed for display, to thereby permit the optimization of the surgical placement and the configuration of lenses.

[0001] This invention was made with United States Government supportfrom the National Institutes of Health (NIH) under Grant No. EY01212.The United States Government has certain rights in the invention.

FIELD OF INVENTION

[0002] This invention relates to a method of collecting, processing anddisplaying data that is generated during the scanning of the eyeutilizing high-frequency ultrasound scanning apparatus and associateddevices.

BACKGROUND OF THE INVENTION

[0003] Information obtained by high-frequency ultrasound scans of thecritical optical dimensions of the human eye have not been fullyutilized to more accurately depict the geometry and/or the topography ofthe relevant portions of the eye in preparation for lens implantationand/or lens replacement in the case of cataract removal. An improvedmethod of providing measurements with a graphical and/or visual displayof the eye for use by technicians and surgeons in preparation for thesurgery is required. Improved methods of post-surgical evaluation of thepositioning of the lens is also needed.

[0004] It is therefore an object of this invention to provide animproved method of collecting, processing and displaying data andgraphic information derived from ultrasound scanning of the eye to moreaccurately depict the geometry and/or the topography of the eye.

[0005] It is a further object of the invention to provide thisinformation in a format that can be utilized by the surgeon to improvethe positioning of the lens and by those those responsible for providingthe implant lenses so that lens design can be optimized for eachsubject.

[0006] Another object of the invention is to provide data and graphicdisplays in a form that can be utilized to improve the design andmanufacture of lenses that more closely conform to the actual geometryof the subject's eye than are currently available.

[0007] Another important finding is that previous assumptions are notcorrect hat the flatter corneal meridian defined the greatest diameterinternal axis.

SUMMARY OF THE INVENTION

[0008] The method of the invention provides new graphicalrepresentations and measurements, as well as visual displays of coronalsections or segments that represent cross-sectional views of therelevant portion of the eye to permit the selection of a lens having theproper power.

[0009] These visual displays and graphical representations also provideinformation not previously available to determine the geometry of thesulcus and angle. Prior art methodology and surgical procedures assumedthat the relevant portion of the eye was circular so that the particularorientation of the lens was not critical.

[0010] Using the method of data collection and processing of theinvention, the relevant portion can be shown to be of oblateconfiguration and the position of the oblate meridian is preciselydetermined and revealed. This display and information permits surgicalplacement of the lens in the optimum position.

[0011] In a further preferred embodiment of the invention, the number ofmeridional scans is increased to further define the interim margins oredges of the in-plane surface.

[0012] The method of the invention is also used to evaluate anypost-placement hazards due to movement of the lens during and followingsurgery. This adjustment phase may occur over a period extending formore than six-months, during which the lens can move to cause discomfortto the patient and to create a risk of damage to the iris or otherelements of the eye. Utilizing the method of the invention, appropriatecoronal scans gather data across the entire iris, i.e., fromangle-to-angle and sulcus-to-sulcus

[0013] The above objects and other advantages are obtained by the methodof the invention which includes the steps of:

[0014] 1. providing a very high-frequency ultrasound scanning apparatus;

[0015] 2. positioning the patient's eye on which lens implanting and/orreplacement is contemplated in position for scanning;

[0016] 3. scanning the patient's eye to thereby generate datarepresentative of a plurality of angularly spaced meridional coronalsections, or meridians, taken across the entire plane of the anteriorsurface of the eye;

[0017] 4. collecting, storing and processing the coronal scan data toidentify thc longest coronal meridian;

[0018] 5. displaying a graphical plot of the longest coronal meridian;and

[0019] 6. processing data from a predetermined number of other coronalsections and providing a display of their length and position relativeto the longest coronal meridian and a graphic plot and measurements ofthe 3-D conformation of the subject's eye.

[0020] The invention further comprehends a method of producing forvisual display a representation of the geometry or the topography, orboth the geometry and topography of a portion of the eye, of a subjectwhich representation is based on data generated by an ultrasound scan ofthe optical components of the eye, the method comprising the steps of:

[0021] a. providing a very high-frequency ultrasound scanning apparatusthat includes a programmed computer and ancillary data storage device;

[0022] b. positioning the subject's eye relative to the apparatus forscanning;

[0023] c. scanning the subject's eye to thereby generate datarepresentative of a plurality of angularly spaced meridional coronalsections taken across the entire plane of the anterior surface of theeye;

[0024] d. collecting and storing the data obtained in step (c) in theancillary data storage device;

[0025] e. processing the coronal scan data; and

[0026] f. generating for visual display a representation of a portion ofthe geometry or topography, or both the geometry and topography of aportion of the subject's eye.

[0027] The method of the invention also comprehends processing the datafrom the scans to provide a cross-sectional representation of theanterior segment of the eye. From this visual display and the datacollected, information can be derived to provide measurements of the eyeincluding the following:

[0028] 1. sulcus plane depth;

[0029] 2. angle-to-angle width; and

[0030] 3. sulcus-to-sulcus width.

[0031] The method of the invention has the advantages of providinggreater accuracy in determining the lens plane position and results in amuch better evaluation of the correct lens power required and itsplacement during surgery.

[0032] The method of the invention also comprehends providing data intabular or graphic form of coronal dimensions that include theangle-to-angle and the sulcus-to-sulcus measurements through 360°. Thisthree-dimensional evaluation permits the largest diameter to bedetermined and its dimension to be precisely ascertained. As a result,the lens haptics, as well as the shape and overall conformation, can becorrectly sized for the largest diameter of the coronal ring, as well asany irregular marginal configurations thereby preventing “propellering”of the lens following surgery.

[0033] The method of the invention can also be utilized to prepare forlens replacement procedures. Because cataractous lenses are usuallyenlarged, preoperative measurements of the anterior chamber depth do notreflect post-operative anterior chamber depth, and hence, proper lensdepth. However, because the implant is placed in the sulcus plane,preoperative measurement of sulcus plane depth in accordance with themethod of the invention provides a basis for an accurate evaluation ofthe postoperative positions of the optically refractive elements of theeye. Fire operative measurements of the sulcus plane dimensions are usedto insure appropriate sizing and positioning of the implant lensessubsequent to removal of a cataractous lens. The data collected ispressed utilizing appropriate software for that purpose. The resultinginformation in the form of data or a graphic display, is utilized toprepare a prescription for the implant lens power that is based oncorneal curvature, axial length and sulcus depth.

[0034] The availability of this data will also allow cylindercorrections to be included in the lens design and provide forimprovement in haptic design. This new dimensional information and theability to record accurate anterior chamber depth measurements for theplacement of the lens provides the further specific advantage of greaterprecision in determining the most appropriate power for the lens. Thecomputation of this corrective lens information utilizes variations ofthe traditional Colenbrander formula and others, and also permits animproved evaluation of existing lens power for greater accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The method of the invention and the results of its applicationare illustrated in the attached drawings, in which

[0036]FIG. 1 is a schematic side elevation view, partly incross-section, showing the optical elements of the eye relative to thescanning apparatus;

[0037]FIG. 2 is a representative cross-sectional visual display preparedfrom the data obtained during an ultrasound scan;

[0038]FIGS. 3A and 3B are plan view illustrations of the eye graphicrepresentations of an angle plane prepared in accordance with the methodof the invention; and

[0039]FIGS. 4A and 4B are, respectively, schematic cross-sectional viewsof the optical elements of an eye before and after surgical removal of acataractous lens.

[0040]FIG. 5 is a plan view illustrating another preferred embodiment ofthe invention;

[0041]FIG. 6 is a graphic representation of trans-axial variationobtained from one preferred embodiment; and

[0042]FIG. 7 is a schematic plan view of the eye showing alternativemethod of scanning.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] In the practice of the method, a very high-frequency ultrasoundsystem 10 is utilized to precisely determine the position andconfiguration of the optical components of the human eye 15 by utilizinga frequency in the 50 megahertz range for the anterior segment and inthe 20 megahertz range for axial length measurement.

[0044] Scans are obtained by first coupling the eye 15 to the transducer10 using a fluid coupling medium, such as normal saline solution. Theregion of interest is then placed in or near the focal plane of thetransducer by adjusting the range of the transducer from the eye. Asshown in FIG. 1, during scanning, the transducer is ranged so that itsfocal plane is in or near the region of interest, i.e., the angle planeor the sulcus plane. The transducer 10 is moved as indicated by thearrow 11, and a series of range-gated pulse/echo signals 12 are obtainedfrom one pole of the eye to the other. The dashed box 14 indicates theregion from which data are obtained. The specific measurementscalculated are identified in FIG. 1 as the angle-to-angle width 21, thesulcus-to-sulcus width 22 and the sulcus plane depth and the angle planedepth 23.

[0045] Ultrasound data are acquired using a focused transducer withcenter frequency of 30 MHz or more, e.g., about 50 MHz for the anteriorsegment; a frequency in the 20 MHz range can be used for axial lengthmeasurement. The higher frequency provides sufficient spatial resolutionto obtain the necessary measurements required for accurate lens implantsizing, positioning and power determination. The transducer ismechanically moved across the eye. The scan plane follows a meridian ormedial line passing through the center of the pupil.

[0046] During movement of the transducer, a series of acoustic pulsesare emitted and echoes digitized, such that pulses are emitted atdistances less than the transducer's focal zone lateral dimension,usually defined as λL/D, where λ is wavelength, D is transduceraperture, and L is focal length. The digitized data are then used togenerate an image of the scan plane. The graphical representation ofFIG. 2 is generated by displaying the angle-to-angle or sulcus-to-sulcusdimensions for a plurality of meridians passing through the center ofthe pupil 24 and then using conventional software to complete theparticular outline depicting the geometry of the oblate shape of theangle plane 21 or sulcus plane 22. A series of scans are made in aplurality of planes so that the planes are angularly equidistant andprovide complete angular coverage of the eye. In a preferred embodimentat least six scan meridian measurements are made at 30-degree intervals,where the planes are in the 12-6 o'clock (vertical with respect to thesubject in a heads-up, forward-looking position). 1-7 o'clock 2-8o'clock, 3-9 o'clock (horizontal), 4-10 o'clock and 5-11 o'clockpositions.

[0047] Dimensional information is recorded for each scan plane, e.g.,angle-to-angle or sulcus-to-sulcus, by use of appropriate software forthis purpose. The development of the lens power and position from thesoftware is well within the capabilities of a trained programmer ofordinary skill in the art. Specific measurements to be obtained in thepractice of the invention are the anterior chamber depth, theangle-to-angle width, the sulcus-to-sulcus width, and the depth of thesulcus plane. These measurements cannot be obtained using conventionaloptical systems due to the opacity of the sclera and iris. The method ofthe invention provides advantages over other radiologic techniques suchas MRI or CT that are both more expensive than ultrasound and providelower spatial resolution.

[0048] A plurality of dimensions of the angle-to-angle width areobtained and the data recorded for use in sizing and placement ofanterior chamber lens implants. Implants consist of an optic (the lensitself) and haptics which are arms extending from the optic that areprovided to keep the lens centered on the pupil. Implants that are toolarge can cause the haptics to press against delicate adjacent tissuewith resultant damage. Implanted lenses that are too small may fall outof position. In addition, because angle-to-angle width may not be thesame at every meridian, the angle plane may describe an ellipse ratherthan a circle. Measurements of angle-to-angle dimensions along with aplurality of medial planes, or meridians, provides information for alens prescription that is appropriate for the eye's dimensions. Inaddition, if the angle plane is elliptical rather than circular, thelens can be sized and implanted appropriately for the largest meridianlength, which will prevent tissue damage or displacement by“propellering.”

[0049] Referring to FIG. 3A, there is depicted a schematic illustrationof the eye showing the pupil 30 (center) surrounded by the iris 32 andthe angle-plane 34. The angle plane is not optically visible due to thepresence of the opaque sclera, or white, of the eye. With reference toFIG. 3A, the lines a-a through f-f indicate scan meridians and biometricmeasurements of angle-to-angle width on six meridians in which the angleplane describes a circle, i.e., the angle-to-angle width is constant atall meridians. In the illustration of FIG. 3B, although the iris andpupil are round, the angle plane describes an ellipse, with its maximumdimension on the 1-to-7 o'clock medial line b-b.

[0050] For implantation of phakic lenses, i.e., implants placed betweenthe crystalline (natural) lens and the iris, measurement ofsulcus-to-sulcus dimensions are used in a method that is analogous tothat described above. As shown in the illustration of FIG. 4A, thecataractous lens 40 is enlarged, resulting in shallowing of the anteriorchamber 41 as indicated by the vertical arrow 42. As shown in FIG. 4B,after extraction of the cataractous lens, one surgical option is toplace the implant lens 44 in the sulcus plane defined by arrow 45 inFIG. 4A where its position is maintained by haptics 46 that are placedin the sulci. Although the depth of the anterior chamber 41 changes withcataract extraction, the sulcus plane depth 48 remains constant.Preoperative measurement of sulcus plane depth 48 will allow calculationof the optimum post-operative implant position and appropriate lenspower for lens 44.

[0051] The method of the invention provides the alternative ofimplanting a lens that can be accommodated and placed in the capsularbag that remains after extraction of the cataractous crystalline lens40. The dimensions of the anticipated capsular bag following surgery armcalculated from the pre-operative measurement of the surface area of thecapsule (lens), thereby permitting the haptic size to be optimized tothe area of the flattened capsular bag.

[0052] A further preferred embodiment of the invention is illustrated inFIG. 5 where the density of medial scans is greater than the sixdescribed above. The setup of the equipment and processing of the datais substantially the same as described above, with the exception that ahigher density of medial scans is undertaken. The additional scansprovide data that is utilized to more precisely define non-ellipticalin-plane surfaces, as are schematically represented in FIG. 5. Thelimitation on the number of scans is determined only by the equipmentand the ability of the subject to maintain a steady position.

[0053] In a particularly preferred embodiment, a fellow eye trackingapparatus of the type used, e.g. in lasik surgical procedures, isemployed during the ultrasound scanning. As will be understood by one ofordinary skill in the art, after a few seconds of scanning the subject'seyes will generally not remain stationary. Where is the number of scansis increased to 20, 40 or even 180, such eye movement is inevitable. Thefellow eye tracking system will take account of any such movement inplotting the data.

[0054] Referring to FIG. 6, there is shown schematically a graphicrepresentation of the anterior-to-posterior variation mapping of theeye. This form of mapping can be provided when the scanning is performedin the high-density mode. With the scanning apparatus in thehigh-density mode, angle or sulcus in-plane, (e.g., across the eye), andtrans-axial (from the front to the back of the eye) variability can bemapped with splines, B-splines or other formulations.

[0055] individual sector scans which extend only between the outerconcentric dotted lines. The intermediate concentric ring formed ofbroken lines represents a plot of the outermost points along the marginof the plane being plotted.

[0056] Because of the relatively short length of each individual scan inthe margin sector, they are of much shorter duration than the scansdescribed above in connection with FIGS. 3A, 3B and 5 that traverse theentire width of the respective portions of the eye.

[0057] In the practice of this method, a mechanical sector scanner orbeam-steered array is coupled to the arc scanner. In this embodiment,the scanner acquires data in a single circumferential path, indicated bythe direction of the arrow, which in the illustration of FIG. 7 isclockwise.

[0058] The marginal sector scanning method described above andillustrated in FIG. 7 can also be utilized to scan a particular marginalsector, e.g., a 30°-60° arc, as distinguished from the entire 360°marginal edge of the desired plane or planes. For example, the eventthat the data obtained from a series of six scans described inconnection with FIGS. 3A and 3B, above, indicated a significant anomalyin one sector, that sector alone can be subjected to a marginal scan toreveal its conformation in more detail. This method can thus bepracticed after the more limited number of meridional coronal scans havebeen completed and the data processed and displayed, and while thesubject is still in position relative to the apparatus.

[0059] The successful use of toric intraocular lens implants forcorrection of astigmatism in both the intact and post-intracapsularcataract extract eye, is dependent on precise lens alignment with theastigmatic axis. Lens propellering, a common post-surgical complicationnecessitating surgical revision, is caused by misalignment of theintra-ocular lens haptic supports and at the conformal surface of theangle or the sulcus. In accordance with the invention, measurements ofthe elliptical surface of the angle and sulcus confirms theirrelationship with the axis of accommodation.

[0060] Using the data from six hemispheric scans, anamorphicallycorrected biobetric angle-to-angle and sulcus-to-sulcus measurements aremodeled using the direct least-squares method. The data is constrainedto an ellipse. Additional scans can be performed, e.g., up to twentyscans, or more, in order to obtain a more precise depiction of theperimeter of the angle surface. The data is evaluated utilizing standardcircular and directed statistic techniques.

[0061] When the method of the invention is utilized to determine thesemi-major axis, it was found to be more accurate than the refractive orthe keratometrically determined axis of astigmatism. Thus, the methodhas utility in providing a more accurate correction to subject's visionthrough both the characteristics of the corrective lens and theplacement of the lens in the subject's eye.

[0062] As will be understood by those of ordinary skill in the art, themethod of the invention is utilized to obtain post-operativemeasurements in order to determine whether any long-term hazards existto the iris, i.e., glaucoma hazard, or to the lens, i.e., cataracthazard.

[0063] As will be apparent to one of ordinary skill in the art from theabove descriptions of the preferred embodiments, the higher resolutionand particularly the three-dimensional representations produced by thepractice of the method of the invention provide greater accuracy in thedetermination of the appropriate lens power for post-cataract surgicalcorrection and for lens implantation in phakic eyes of patients.

We claim:
 1. A method of producing for visual display a representationof the geometry or the topography, or both the geometry and topographyof a portion of the eye of a subject which representation is based ondata generated by an ultrasound scan of the optical components of theeye, the method comprising the steps of: a. providing a veryhigh-frequency ultrasound scanning apparatus that includes a programmedcomputer and ancillary data storage device; b. positioning the subject'seye relative to the apparatus for scanning; c. scanning the subject'seye to thereby generate data representative of a plurality ofangularly-spaced meridional sections taken across the entire coronalplane of the anterior surface of the eye; d. collecting and storing thedata obtained in step (c) in the ancillary data storage device; e.processing the coronal scan data; and f. generating for visual display arepresentation of a portion of the geometry or topography, or both thegeometry and topography of a portion of the subject's eye.
 2. The methodof claim 1, wherein the processing of the coronal scan data includes thesteps of: identifying the longest coronal meridian and a plurality ofother coronal meridians in one or more optical planes; and generating agraphic display of the coronal meridians indicating their relativelengths across the one or more optical planes.
 3. The method of claim 1,wherein the processing of the coronal scan data includes the steps of:generating meridional measurements that includes one or morecharacteristics selected from the sulcus plane depth, the angle-to-anglewidth and the sulcus-to-sulcus width.
 4. The method of claim 3, whereinthe visual representation includes all three of the characteristics. 5.The method of claim 1, wherein the representation is printed for visualdisplay.
 6. The method of claim 1, wherein the representation isvisually displayed on a monitor.
 7. The method of claim 1, wherein atleast six angularly spaced coronal section scans are performed.
 8. Themethod of claim 7, wherein multiple equally-spaced scans are performed.9. The method of claim 1, wherein the angular displacement between thescans are approximately equal.
 10. The method of claim 9, where theangular displacement between each of the scans is from 30° to 1°. 11.The method of claim 1, which further comprises: providing a fellow eyetracking device; operatively connecting the fellow eye tracking deviceto the ultrasound scanning apparatus and the data storage device; andpositioning the eye of the subject that is not being scanned relative tothe tracking device.
 12. The method of claim 11, wherein the angulardisplacement between each scan is 20° or less.
 13. A method of producingfor visual display a representation of the geometry or the topography,or both the geometry and topography of a portion of the eye of asubject, which representation is based on data generated by anultrasound scan of the optical components of the eye, the methodcomprising the steps of: a. providing a very high-frequency ultrasoundscanning apparatus that includes a programmed computer and ancillarydata storage device; b. positioning the subject's eye relative to theapparatus for scanning; c. scanning the subject's eye to therebygenerate data representative of a plurality of angularly-spaced sectionstaken across a marginal portion of the coronal plane of the anteriorsurface of the eye; d. collecting and storing the data obtained in step(c) in the ancillary data storage device; e. processing the sectionalscan data; and f. generating for visual display a representation of aportion of the geometry or topography, or both the geometry andtopography of a portion of the subject's eye.
 14. The method of claim13, wherein the margin portion is defined by a pair of concentricboundaries disposed on either side of the terminii of the maximumcoronal meridian of the eye of the subject.